1. Field of the Invention
The present invention relates to apparatus for amplifying electrical signals. More particularly, this invention pertains to an improved field effect transistor ("FET") amplifier.
2. Description of the Prior Art
Numerous devices such as receivers for missiles and the like that present extreme space limitations require amplifiers of the wideband solid-state microwave gallium arsenide FET type. In addition to space requirements, such amplifiers must meet difficult-to-attain specifications with regard to small-signal gain, linearity and gain compensation stability as a function of temperature.
One method that has been attempted to achieve a high small-signal gain per stage is disclosed in FIG. 1. As is seen, this involves the connection of identical FET devices 10, 12 in a feedback configuration. While such an arrangement does in fact increase the small-signal gain, it is hampered by the known temperature-dependence of FET gain. That is, the gain of the FET decreases as temperature is increased. This poses a serious design problem and a common solution is to cascade a gain stage with a temperature-varying PIN diode attentuator.
Unfortunately, the use of a PIN diode attenuator is hampered by the (approximately 5 dB) insertion loss of the attenuator which lowers overall amplifier gain. Further, the insertion loss caused by the attenuator tends to worsen the amplifier noise figure and to lower the output power of the temperature-compensated cascade. Finally, the above-mentioned crucial size constraints imposed by various applications are jeopardized by the addition of the attenuator which is often at least as large as the amplifier arrangement.